1. Field of the Invention
The present invention relates to a semiconductor laser chip unit for use in optical fiber communications, and a semiconductor laser module using the semiconductor laser chip unit.
2. Description of the Related Art
As the use of the Internet has been explosively widespread, demand for expanding transmission capacity is strongly increased in recent years. Thus, not only an optical transmission using the wavelength of a single semiconductor laser diode, but also it is important to realize wavelength multiplex communications in which a plurality of semiconductor laser diodes are used and a plurality of wavelengths are multiplexed with high density. In the case of the multiplex communications, since a semiconductor laser diode has such a characteristic that the oscillation wavelength thereof fluctuates depending on temperatures (eg., 0.1 nm/° C.), a technique to keep the temperature of the semiconductor laser diodes constant is known. However, only keeping the temperature of the semiconductor laser diode constant cannot suppress with time of the semiconductor laser diode.
In the Japanese Patent Application Laid-open No. 2001-313613, for example, there is disclosed a technique to continuously detect the wavelength of a semiconductor laser diode and to feedback-control the temperature of the semiconductor laser diode so as to keep the wavelength constant. FIG. 1 is a diagram showing a conventional semiconductor laser module disclosed in the Japanese Patent Application Laid-open No. 2001-313613. Explanations will be given below with reference to the Figure.
In FIG. 1, a light source 111 for use in the semiconductor laser unit is an integrated light source in which a semiconductor laser chip 101 of the distributed feedback type and an optical modulator 102 of the electric field absorption type are monolithically integrated. A front optical output from the integrated light source 111 is optically coupled on an optical fiber 115 through a front collimator lens 112, an optical isolator 113 which blocks a reflected return light from an optical module on the transmission path, and a convergent lens 114.
A back optical output from the integrated light source 111, on the other hand, is made incident on a high reflection film 104 coated on the light receiving surface of a photodiode 103 through a back collimator lens 117. By setting the reflection rate of the high reflection film 104 to be about 50 to 80%, a portion of the back optical output from the integrated light source 111 is made incident on the photodiode 103 and the remaining portion is made reflected onto a wavelength filter 118. This photodiode 103 serves as an optical output monitor 116.
The laser beam reflected at the high reflection film 104 of the optical output monitor 116 is made incident on a non-reflective film 105 coated on the light receiving surface of a photodiode 106 through a wavelength filter 118, the transmission loss of which has a wavelength dependency. This photodiode 106 serves as a wavelength monitor 119.
In this semiconductor laser unit, respective optical elements, including the integrated light source 111, the front collimator lens 112, the back collimator lens 117, the optical output monitor 116, the wavelength filter 118 and the wavelength monitor 119, are mounted on a stem 122 on which a temperature detection element 120 is mounted, which stem is disposed on a thermionic cooling element 121. Therefore, respective optical elements are kept at the stable temperature and are fixed in a mechanically stable manner within a hermetic sealing package 123 by soldering or YAG laser welding.
In this way, the back optical output from the integrated light source 111 is branched. One of the branched output is detected at the photodiode 103, and the other is passed through the wavelength filter 118 so as to be detected at the photodiode 106. Based on signals from the photodiode 106 reflecting the transmission characteristic of the wavelength filter 118, the oscillation wavelength of the semiconductor laser diode is controlled by feedback-controlling the temperature of the integrated light source 111 with a use of the temperature detection element 120 and the thermionic cooling element 121.
However, in the technique disclosed in the Japanese Patent Application Laid-open No. 2001-313613, it is required that, in the state of the final product that an optical unit is hermetically sealed by the hermetic sealing package 123, the high-frequency characteristic of the semiconductor laser chip 101 be measured by inputting a high-frequency signal with a use of a pin (not shown) of the hermetic sealing package 123. If the semiconductor laser chip 101 is determined as defective in this measurement stage, the hermetic sealing package 123 and each optical unit must be disposed, or the unit must be replaced spending many work man-hours.
In another Japanese Patent Application Laid-open No. 2001-164970, there is disclosed a structure in which a semiconductor laser and a coplanar line are provided on a substrate which is mounted on a career. With this structure, the high-frequency characteristic of a semiconductor laser diode can be measured on the way of the assembling step of the semiconductor laser module, that is, before mounted on the career. However, since it has a structure that the back optical output from the semiconductor laser chip is directly received at a photodiode, a precise optical axis adjustment is required between the semiconductor laser chip and the photodiode.